Ultrasound transducer assembly and method for manufacturing an ultrasound transducer assembly

ABSTRACT

The present invention relates to an ultrasound transducer assembly (10), in particular for intravascular ultrasound systems. The ultrasound transducer assembly comprises at least one silicon substrate element (30) including an ultrasound transducer element (14) for emitting and receiving ultrasound waves and including electrical connectors for electrically connecting the transducer element. The substrate element has a top surface (44), a bottom surface (46) and a side surface connecting the top surface and the bottom surface. An isolation layer (32, 50) forms the side surface for electrically isolating the substrate element.

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2015/054301, filed on Mar.2, 2015, which claims the benefit of European Application Serial No.14159036.4, filed Mar. 12, 2014. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an ultrasound transducer assembly, inparticular a capacitive micro-machined ultrasound transducer (CMUT) cellfor intravascular ultrasound transducer and a method of manufacturingthe same. The present invention relates further to ultrasoundtransducer.

BACKGROUND OF THE INVENTION

In the field of intravascular ultrasound devices, it is generally knownto mount an ultrasound transducer on the tip of a catheter to form aradial ultrasound image e.g. of a blood vessel or the surroundingtissue. The ultrasound transducer elements may be rotated in order toemit and receive ultrasound waves in the radial direction of thecatheter.

It is further known to replace the mechanically scanning ultrasoundtransducer elements in the intravascular ultrasound devices byelectronically scanning devices having an annular array of ultrasoundtransducer elements, which are usually formed by piezoceramictransducers and which may be replaced in future applications bycapacitive micro-machined ultrasound transducers (CMUT). Thesecapacitive micro-machined ultrasound transducers are manufactured on thebasis of a silicon wafer by means of IC process technologies and can bemanufactured with low costs and can be scaled down to the dimensions ofan intravascular ultrasound transducer.

For manufacturing an annular ultrasound transducer, a semi-flexibleultrasound transducer array is formed on the basis of a silicon wafersubstrate that is optionally wrapped or bent around and attached to acylindrically shaped submount structure in order to form the annulararray and to transmit and receive ultrasound transducer waves in theradial direction of a catheter. These semi-flexible ultrasoundtransducer arrays consist usually of several silicon elements comprisingultrasound transducer elements which are connected to each other by aflexible element, so that the rigid silicon elements can be wrappedaround a cylindrically shaped submount structure.

A bendable micro-machined ultrasound transducer array, which can beattached to a cylindrical submount structure to form thecylindrically-shaped ultrasound transducer array is known from US2005/0146247 A2.

An electromechanical transducer including a multiple cellular structureis known from EP 2 441 530 A2, wherein a front silicon film iselectrically connected to a rear surface of the multiple cellularstructure by means of a via plug.

The silicon elements of the ultrasound transducer, which are flexiblyconnected have to be fabricated with high precision and the electricalfunctionality of the integrated ultrasound transducer elements and theelectrical circuits have to be guaranteed in order to assure thefunctionality of the ultrasound transducer array in general.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ultrasoundtransducer assembly, in particular for intravascular ultrasound systems,having an improved reliability and which can be manufactured preciselywith low technical effort.

In a first aspect of the present invention, an ultrasound transducerassembly, in particular for intravascular ultrasound systems isprovided, comprising:

-   -   at least one silicon substrate element including an ultrasound        transducer element for emitting and receiving ultrasound waves        and including electrical connectors for electrically connecting        the transducer element,    -   wherein the substrate element has a top surface, a bottom        surface and a side surface connecting the top surface and the        bottom surface, and    -   wherein an isolation layer is provided, which is formed at the        side surface or which forms the side surface for electrically        isolating the substrate element.

In a further aspect of the present invention, a method for manufacturingan ultrasound transducer assembly, in particular for intravascularultrasound systems, is provided comprising the steps of:

-   -   providing a silicon substrate having a substrate element portion        including at least one ultrasound transducer element for        transmitting and receiving ultrasound waves and including        electrical connectors for electrically connecting the transducer        element,    -   forming a trench in the silicon substrate laterally separating        the substrate element portion from an intermediate substrate        portion surrounding the substrate element portion,    -   filling the trench with a material different from the silicon        substrate material to form a side layer of the substrate element        portion, and    -   removing the intermediate substrate portion surrounding the side        layer to separate the substrate element portion from the silicon        substrate.

In a still further aspect of the present invention, an ultrasoundtransducer, in particular for intravascular ultrasound systems isprovided comprising an elongated probe including a tip and an ultrasoundtransducer assembly of this kind for emitting and receiving ultrasoundwaves.

Preferred embodiments of the invention are defined in the dependentclaims. It shall be understood that the claimed method has similarand/or identical preferred embodiments as the claimed device and asdefined in the dependent claims.

The present invention is based on the idea to manufacture the siliconsubstrate elements, which form a part of the bendable ultrasoundtransducer assembly in a micro-fabrication process on the basis of asilicon substrate which include the ultrasound transducer elements andelectrical circuits for connecting the same. The side surface of thesilicon substrate elements are isolated electrically by means of theisolation layer, which is formed at or forms the side surface of thesubstrate element in order to avoid short circuits. Hence, the siliconsubstrate elements have due to the electrical isolation lessmalfunctions so that the overall reliability of the ultrasoundtransducer assembly is improved. Further, the manufacturing method isbased on the idea to first form trenches in the silicon substrate and tofill the trenches with a material different from the silicon substratematerial to form the substrate elements in the respective substrateelement portion and to remove the intermediate substrate in a followingstep to separate the substrate element from the silicon substrate. Sincethe trenches are formed laterally separating the substrate elementportion from the intermediate substrate portion and the intermediatesubstrate portion is removed in a separate step to separate thesubstrate element from the silicon substrate, the trenches can be formedprecisely by means of an accurate manufacturing step e.g. an anisotropicetch process like DRIE and the intermediate substrate portion can beremoved by a coarse process step e.g. by an isotropic etch process likewet etching. Since the dimensions are defined by the trenches, which canbe produced by a reduced technical effort and since the intermediatesubstrate portion is removed entirely by a coarse process step, theaccuracy of the manufacturing process can be increased with lowtechnical effort. The side surface, which is formed by the isolationlayer forms an outer or exterior surface of the substrate element. Theouter surface can be adjacent to a hole in the substrate element or cansurround the substrate element. This is a possibility to isolate thesubstrate elements from other substrate elements or from a connectionwire.

Consequently, the overall reliability of the ultrasound transducerassembly can be improved and the technical effort for a precisemanufacturing of the ultrasound transducer assembly can be reduced.

In a preferred embodiment, the ultrasound transducer assembly comprisesa plurality of silicon substrate elements separated from each other anda flexible connection layer for flexibly connecting the substrateelements to each other. This is a possibility to achieve a semi-flexibleultrasound transducer assembly which can be manufactured by means of anIC process technology and can be wrapped or bended around a submountstructure in order to form an annular array of transducer elements.

In a preferred embodiment, a top isolation layer is formed at the topsurface or forms the top surface for isolating the silicon substrateelement. This is a possibility to further improve the electricalisolation of the silicon substrate element and to improve the overallreliability of the ultrasound transducer assembly.

In a preferred embodiment, a bottom isolation layer is formed at thebottom surface or forms the bottom surface for isolating the siliconsubstrate element. This is a possibility to improve the electricalisolation of the silicon substrate element in order to improve theoverall reliability of the ultrasound transducer assembly.

In a preferred embodiment, an electrical connection pad is connected tothe flexible connection layer for electrically connecting the transducerassembly to a catheter. This is a possibility to provide an electricalconnection with low technical effort.

In a preferred embodiment, the isolation layer comprises silicon oxideor a polymer material. This is a possibility to provide an isolationlayer which can be manufactured with low technical effort.

In a preferred embodiment of the method, a plurality of substrateelement portions are laterally separated by a plurality of trenches,wherein the intermediate substrate portions between the substrateelement portions are removed to separate the substrate element portionsfrom the silicon substrate. This is a possibility to manufacture aplurality of substrate elements in parallel with the same manufacturingmethod in order to provide an ultrasound transducer assembly having aplurality of silicon substrate elements and a plurality of ultrasoundtransducer elements.

In a preferred embodiment, the trenches are formed vertically from afirst side of the silicon substrate and the intermediate substrateportion is removed from a second side of the silicon substrate oppositeto the first side of the silicon substrate. This is a possibility toutilize different manufacturing processes, which can be applied fromdifferent sides of the silicon substrate, wherein the trenches and thefilled-in material predefine the shape of the substrate elements, whichcan be separated by a coarse manufacturing process from the oppositeside of the substrate.

In a preferred embodiment, the intermediate substrate portion is removedby an etching process and the side layer forms a lateral etch stop layerfor the etching process. This is a possibility to define the substrateelement portion by means of a precise etching process for etching thetrenches and to remove the intermediate substrate portion by a coarseetching process since the substrate element portions are protected bythe etch stop layer.

In a preferred embodiment, a flexible layer is formed at a portionoverlaying the intermediate substrate portion between the two substrateelement portions to form a flexible connection between the substrateelement portions. This is a possibility to flexibly connect thesubstrate elements after the substrate elements are separated from thesilicon substrate by means of the flexible layer which is overlappingthe intermediate substrate portion and connected to a surface of thesubstrate element portions to be connected.

In a preferred embodiment, the silicon substrate comprises a first and asecond silicon layer disposed on top of each other separated by anintermediate layer, wherein the substrate element portion is formed inthe first silicon layer and the intermediate layer forms a vertical etchstop layer. This is a possibility to precisely determine the thicknessof the substrate elements since the thickness is determined by theposition of the etch stop layer.

In a preferred embodiment, the intermediate layer is patterned by anetch mask and an etch process in order to expose the intermediatesubstrate portion surrounding the side layers. This is a possibility toremove the intermediate substrate portion between the side layer withlow technical effort.

In a preferred embodiment, the trench is filled with an isolationmaterial to form the side layer as an isolation layer. This is apossibility to manufacture the substrate elements having a highreliability, since the electrical functional is assured by the isolationof the side surface of the substrate elements.

As mentioned above, the ultrasound transducer assembly has an improvedreliability, since the silicon substrate elements formed on the basis ofa silicon substrate in an IC process are electrically isolated by theisolation layer formed at the side surface so that short circuits andthe related malfunctions can be avoided. Further, since the trenches areformed in the silicon substrate to laterally separate the substrateelement portions and filled with a material different from the siliconsubstrate material, the shape of the substrate elements can be preciselydefined by the trench-etching process and the intermediate substrateportion can be easily removed by a coarse process e.g. an isotropic etchprocess so that the precise shape of the substrate elements can beachieved with low technical effort.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter. Inthe following drawings

FIG. 1 shows a schematic drawing of an ultrasound transducer assembly ina plane view before assembling;

FIG. 2 shows a schematic drawing of a cross section of the assembledultrasound transducer assembly in an axial view;

FIG. 3 shows a cross-sectional view of a processed silicon waferincluding silicon substrate elements forming different ultrasoundtransducer elements of the ultrasound transducer assembly;

FIGS. 4a-l show a sequence of manufacturing steps for manufacturing theultrasound transducer assembly;

FIGS. 5a-e show an alternative sequence of manufacturing steps formanufacturing the ultrasound transducer assembly;

FIGS. 6a-h show an alternative sequence of manufacturing steps formanufacturing the ultrasound transducer assembly; and

FIGS. 7a-s show a sequence of manufacturing steps for manufacturing theultrasound transducer assembly including a horizontal etch stop layer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic top view of an ultrasound transducer assemblygenerally denoted by 10. The transducer assembly comprises a transducerarray 12 including a plurality of transducer elements 14 for emittingand receiving ultrasound waves. The ultrasound transducer assembly 10comprises a support element 16 which serve as a submount element for thetransducer array 12. The support element 16 has a circular shape and isconnected to the transducer array 12 by means of a flexible connectionlayer 20. The transducer array 12 has an elongated shape in thedirection of a longitudinal axis 22.

The transducer elements 14 are formed as capacitive micro-machinedultrasound transducers (CMUT). The transducer elements are flexiblyconnected to each other so that the transducer array 12 can be bent inorder to form an annular, circular or polygonal transducer array asdescribed in the following. The transducer elements 14 can be flexiblyconnected to each other by means of a flexible layer, which may beconnected in one piece to the connection layer 20.

The support element 16 comprises a central opening 24 in order tosupport the transducer assembly 10. The flexible connection layer 20flexibly connecting the support element 16 to the transducer array 12comprises integrated electrical interconnects for electricallyconnecting the transducer elements 14 to the support element 16.

The transducer assembly 10 is made from a silicon wafer by means of amicro-fabrication process using integrated circuit-processing technologyto form the transducer array 12 and the support element 16 as describedin the following. The silicon wafer may be a blank wafer or may includepre-processed active devices or circuits like CMOS transistors and or(high-density) capacitors. The transducer elements 14 may consist ofsilicon islands containing ultrasound transducer elements such as CMUTtransducers, electrical circuits and or capacitors connected to eachother by means of the flexible connection layer 20.

The support element 16 is connected to the transducer array 12 by meansof the flexible connection layer 20 so that the support element 16 canbe bent by 90° and the flexible transducer array can be wrapped aroundthe support element 16 in order to form the transducer assembly 10 in acircular form. The so-formed transducer assembly 10 may be connected toan intravascular ultrasound system for emitting and receiving ultrasoundwaves in a radial direction.

The embodiment shown in FIG. 1 comprises a linear array of transducerelements 14, however any shape and any formation of transducer elements14 is possible and can be used by the present invention, e.g. circularor polygonal transducer elements which are disposed in a one-dimensionalarray or a two-dimensional array including columns and rows oftransducer elements 14 which may be alternatingly displaced.

FIG. 2 shows a schematic cross-sectional view of the ultrasoundtransducer assembly 10 in an axial viewing direction. The transducerelements 14 are wrapped around the multi-phased support portion of thesupport element 16 and respectively attached to the different phases ofthe support portion. The transducer elements 14 are flexible connectedto each other by means of the connection layer 20 which is provided atthe outside of the transducer array 12. By means of the so-assembledtransducer assembly 10 ultrasound waves can be transmitted and detectedin a radial direction of the transducer assembly 10.

The size of the transducer elements 14, which are formed in a siliconsubstrate have a submillimeter size and require a precise shape and acorrespondingly precise manufacturing process and an isolation from eachother in order to provide a reliable functionality.

FIG. 3 shows a sectional view of an ultrasound transducer assembly 10formed by an IC process in a silicon substrate. The transducer assembly10 comprises different silicon substrate elements 30, which may form thetransducer elements 14 including electrical circuits for electricallyconnecting the transducer elements 14. The substrate elements 30 areseparated from each other and flexibly connected to each other by meansof a flexible connection layer 32, which is preferably formed of apolyimide, a parylene or a PDMS layer. The substrate elements 30 areisolated by means of a side isolation layer 34 formed at a side surfaceof the substrate elements 30. The side isolation layer 34 form an outerside surface or an exterior surface of the substrate elements 30. Inother words, the side isolation layer 34 is exposed to the outside orthe exterior. The substrate elements 30 are further isolated by means ofa top isolation layer 36 formed at a top surface and by means of abottom isolation layer 38 formed at a bottom surface of the substrateelements 30. The outer substrate elements 30 are each attached to alower support element 39, which are provided for supporting thesubstrate elements 30 radially after bending. On the top surface of thepolyimide layer 32 connection pads 40 are connected for connecting theultrasound transducer elements 14 to a catheter core wire.

The central substrate elements 30 may form the support element 16 havingthe central opening 24 to support the ultrasound assembly 10. Thepolyimide layer 32 serves to flexibly connecting the substrate elements30 to each other so that the substrate elements 30 can be bent in orderto form the cylindrical shape of the assembled ultrasound transducerassembly 10 as shown in FIG. 2.

The side isolation layer 34 serves in particular to isolate thesubstrate elements 30 electrically from each other so that shortcircuits can be avoided and the reliability of the ultrasound transducerassembly 10 can be improved. The side isolation layer 34 further servesas vertical etch stop layer in order to improve the preciseness of themanufacturing process as described in the following.

In FIG. 4a-l manufacturing steps for manufacturing an embodiment of theultrasound transducer assembly 10 from a silicon substrate are shown.

In FIG. 4a a silicon substrate is shown having a thickness in the orderof 500 μm as a basis for the manufacturing process. The siliconsubstrate is generally denoted by 42. The silicon substrate 42 comprisesa front side 44 and a back side 46. In a first step, shown in FIG. 4a ,vertical trenches 48 are formed from the front side 44 in the siliconsubstrate 42. The trenches 48 have a width in the order of 1-5 μm and atypical depth in the order 50 μm. The trenches 48 are formed by ananisotropic etch process e.g. a DRIE etch process. The trenches 48laterally separate the substrate elements 30, which are formed by meansof the manufacturing process.

In a following step shown in FIG. 4b an etch stop layer 50 is depositedon the front side 44 which fills the trenches 48. The etch stop layermay be formed by oxide, nitride or polymer like polyimide,benzo-cyclobutene and parylene.

In a following step (FIG. 4c ), the etch stop layer 50 is etched back sothat only the trenches 48 remain filled with the etch stop layer 50.

In the following step shown in FIG. 4d a etch-stop layer 52 is depositedon the front side 44 and a hard-mask layer 54 is deposited on the backside 46.

In the following steps shown in FIGS. 4e-g the hard mask 54 on the backside 46 is patterned at an intermediate substrate portion 56 between thetrenches 48, the etch stop layers 50 and between the substrate elements30. The intermediate substrate portions 56 have to be removed in orderto separate the substrate elements 30 as described in the following. Ina following step shown in FIG. 4f , a photoresist 58 is formed on theback side 46 and the back side 46 is etched so that trenches 60 areformed at a back side of the intermediate portions 56. In a followingstep, the hard mask 54 is removed from the back side 46 as shown in FIG.4h . Hence, the back side 46 of the silicon substrate 42 is exposedincluding the trenches 60 at the position corresponding to theintermediate portions 56.

In a following etch process step, the back side 46 is etched in ananisotropic etch process so that the trenches 60 are etched down to theintermediate portions 56 as shown in FIG. 4i . In this state, a layerbetween the trenches 60 and the etch stop layers 50 is remaining havingtypically a thickness of 3-5 μm. At this point, the etching process isswitched to an isotropic etching process. The silicon of the siliconsubstrate 42 will be etched in the vertical and in the lateral directionso that the remaining layer between the trenches 60 and etch stop layers50 is entirely removed as shown in FIG. 4j . In this state, theintermediate portions 56 are entirely removed and the etch stop layers50 are exposed laterally at one side. Due to the etch stop layer 50, theisotropic etch process does not affect the lateral side of the substrateelements 30, since the etch stop layer 50 protect the substrate elements30 from being laterally etched. Hence, the precision of the trenches 48formed by the anisotropic precise etching process for the front side 44determine the lateral size of the substrate elements 30. Further, theback side 46 can be etched by a coarse isotropic etch process so thatthe technical effort for etching the back side 46 can be reduced.

In the following step, the etch-stop layer 52 and the hard mask 54 areremoved so that the substrate elements 30 comprising the etch stop layer50, which are identical to the side isolation layers 34 remain as shownin FIG. 4 l.

In FIG. 5a-e process steps are shown for manufacturing the ultrasoundtransducer assembly 10 including the flexible connection layer 32 forflexibly connecting the substrate elements 30 to each other. Thisprocess roughly follows the process shown in FIG. 4 after the etch stoplayer 50 is formed at the front side 44 of the silicon substrate 42.Identical elements are denoted by identical reference numerals, whereinhere merely the differences are explained in detail. In FIG. 5a apolyimide layer 62 is deposited onto the front side 44 and onto the etchstop layer 52. The polyimide layer 62 may be just a layer of planepolyimide or multiple layers containing interconnects which electricallyconnect the different substrate elements 30 to each other in order toprovide an electrical connection to the transducer elements 14. Thepolyimide layer 62 may alternatively be formed of any other polymer likeparylene, PDMS or the like.

Before the etching of the back side 46, aluminium hard-mask layer 64 isdeposited and patterned on the polyimide layer 62. The hard-mask layer64 is formed over the intermediate portions 56 and is overlapping withthe substrate elements 30 correspondingly and is overlapping with therespective etch stop layer 50 in order to form the flexible connectionlayer 32 between the substrate elements 30 as described in thefollowing. After the deposition of the hard-mask layer 64, the back sideis etched in order to remove the intermediate portions 56 as shown inFIG. 5c , wherein the etch stop layer 50 forms lateral etch stops forthe etching process and determines the shape of the substrate elements30 as described above.

In a following process step, the etch stop layer 52, which is preferablya silicon oxide layer is etched from the back side through the openingformed between the etch stop layers 50 as shown in FIG. 5d .Consequently, the substrate elements 30 are in this state merelyconnected to each other by means of the polyimide layer 62. In a finalstep, the polyimide layer 62 on the front side 44 is etched in an oxygenplasma using the aluminium hard-mask layer 64 on the front side 44 as ahard-etch mask so that the polyimide layer 62 merely remains below thehard-mask layer 64 and flexibly and, if applicable, electricallyconnects the respective substrate elements 30 as shown in FIG. 5 e.

In FIGS. 6a-h process steps are shown for forming the electricalconnection pads 40 on the front side 44 on top of the flexible layer 32.The process is comparable to the process shown in FIG. 5. Identicalelements are denoted by identical reference numerals, wherein heremerely the differences are explained in detail. The process starts afterthe deposition of the polyimide layer 62 shown in FIG. 6a , which isidentical with FIG. 5 a.

The electrical connection pads 40 may be deposited on the polyimidelayer 62 around intermediate portions 66, which forms a hole 24 in thesupport element 16 in which a tip of a connection wire can be inserted.The electrical connection pads 40 are intended for electrical connectionof the transducer elements 14. The connection pads 40 are formed aroundthe hole 24, which is in this case formed as a central hole 24, however,the hole 24 may be formed at any position separate from the centre ofthe ultrasound assembly 10. Further, the ultrasound assembly 10 maycomprise a plurality of connection pads 40 formed at different positionsand/or around different holes in order to form an electrical connection.The central hole 24 may have other functions than an electricalconnection, e.g. forming a mechanical support portion.

In a following step a hard-mask layer 68 is deposited and patterned ontothe connection pads 40 and is formed preferably of aluminium as shown inFIG. 6 c.

In the following steps shown in FIGS. 6d and e , the substrate 42 andthe etch stop layer 52 are etched from the back side 46 as describedbefore.

In a following step, the polyimide layer 62 is etched by means of theoxygen plasma from the front side so that the polyimide layer 62 whichis not covered by the hard mask 68 is removed as shown in FIG. 6 f.

In a following step shown in FIG. 6g , the aluminium layer as thehard-mask 68 is removed so that the substrate elements 30 are connectedto each other by means of the flexible layer 32 and the connection pads40 are connected to the flexible connection layer 32.

A connection wire 70 may be inserted into the opening 24 and soldered tothe connection pads 40 in order to form an electrical connection to thesubstrate element and to support the transducer elements 14. Thesubstrate elements 30 on the outer side can be bent by means of theflexible connection layer 32 so that a cylindrical shape can be formedaround the wire 70.

In FIGS. 7a-s manufacturing steps are shown for manufacturing theultrasound transducer assembly 10, wherein the thickness of thesubstrate elements 30 is precisely determined with low technical effort.

In FIG. 7a , the substrate 42 is provided, wherein on the front side 44an isolation layer 72 and a silicon layer 74 are formed. Hence, thesubstrate comprises the top silicon layer 74, the isolation layer 72 andthe bottom silicon layer, which is corresponding to the siliconsubstrate 42. In the following steps shown in FIG. 7b-d , the etch stoplayer 50 are formed by etching and filling the trenches 48 in the topsilicon layer 74 as described above. In this step, the isolation layer72 forms as an etch stop layer for the vertical etching of the trenches48.

In a following step, the hard-mask layer 54 is formed at the back side46 and a spin coating and curing of a polyimide layer 76 is performed onthe front side 44 in order to form a 10 μm layer of polyimide 76.

In the following step, the connection pads 40 are formed around theintermediate portion 66, where the opening 24 of the support element 16will be formed. The connection pads may be formed as a ring around theintermediate portion 66 e.g. by means of a sputter process and apatterning, wherein the connection pads 40 are preferably formed of a 1μ, thick layer of aluminium. Before etching the back side 46, thealuminium hard-mask layer 68 is deposited and patterned on the frontside 44 as described above.

In a following step, the hard mask 54 is patterned at the intermediateportions 56 so that the silicon substrate 42 is exposed at theintermediate portions 56.

In the following steps shown in FIGS. 7i and j , the photoresist 58 isformed at the back side 46 and the trenches 60 are etched in the siliconsubstrate 42 as described above.

In a following step shown in FIGS. 7k and l , the hard mask 54 isremoved from the back side 46 and the silicon substrate 42 is etched inan anisotropic etch process preferably a DRIE etch process so that thetrenches 60 are extended to the isolation layer 72 as shown in FIG. 7 l.

In a following step shown in FIG. 7, the isolation layer 72 is etched atthe exposed positions corresponding to the trenches 60, wherein theremaining silicon substrate beside the trenches 60 is used as a masklayer.

In a following step shown in FIG. 7n , the remaining silicon of theintermediate portion 56 is etched by an DRIE etch process so that alayer of silicon is remained on the etch stop layers 50 as explainedabove.

The etching of the back side 46 is completed by an isotropic etch toremove the remaining silicon as describe above and as shown in FIG. 7o .The laterally etch stop is provided by the etch stop layer 50 and theisolation layer 72 forms and etch stop layer in the vertical directionso that the thickness of the substrate elements 30 is determined by theisolation layer 72.

In the following steps the polyimide layer 76 is etched by means of anoxygen plasma as shown in FIG. 7p and the aluminium hard-mask layer 68is removed in the step shown in FIG. 7q . Hence, the substrate elements30 are separated from each other and flexibly connected by means of theflexible connection layer 32 and connected to the connection pad 40,which may be formed as a ring around the hole 24. In FIG. 7r , the wire70 is connected to the connection pads 40 in order to form an electricalconnection to the ultrasound transducer elements 14 and/or in order toform a mechanical connection to the transducer elements 14.

Finally, the substrate elements 30 at the two sides may be bended to thewire 70 in order to form the cylindrical shape of the transducerassembly 10. Alternatively, the substrate elements 30 around the centralhole which are connected to the wire 70 may be separated from theremaining substrate elements 30 by breaking the flexible layer 32 asshown in FIG. 7 s.

Finally, the etch stop layer 50 forms the side isolation layer 34 andthe isolation layer 72 which forms during the process the vertical etchstop layer forms the bottom isolation layer 38 and the polyimide layer76 or an additional isolation layer forms the top isolation layer 36.

Hence, the ultrasound transducer assembly can be manufactured preciselywith a precise shape of the substrate elements 30 with low technicaleffort and the substrate elements 30 can be isolated from each other bylow technical effort so that the reliability of the ultrasoundtransducer assembly 10 is improved.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single element or other unit may fulfill the functions ofseveral items recited in the claims. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

The invention claimed is:
 1. An ultrasound transducer assembly forintravascular ultrasound systems, comprising: a silicon substrateelement including an ultrasound transducer element for emitting andreceiving ultrasound waves and including electrical connectors forelectrically connecting the ultrasound transducer element, wherein thesilicon substrate element has a top surface, a bottom surface and a sidesurface connecting the top surface and the bottom surface to each other,and wherein an isolation layer forms the side surface connecting the topsurface and the bottom surface to each other for electrically isolatingthe silicon substrate element, the isolation layer forming an exteriorsurface of the side surface of the silicon substrate element.
 2. Anultrasound transducer assembly as claimed in claim 1, comprising aplurality of silicon substrate elements separated from each other and aflexible connection layer for flexibly connecting the substrate elementsto each other.
 3. An ultrasound transducer assembly as claimed in claim1, wherein a top isolation layer is formed at the top surface forisolating the silicon substrate element.
 4. An ultrasound transducerassembly as claimed in claim 1, wherein a bottom isolation layer isformed at the bottom surface for isolating the silicon substrateelement.
 5. An ultrasound transducer assembly as claimed in claim 2,wherein an electrical connection pad is connected to the flexibleconnection layer for electrically connecting the transducer assembly. 6.An ultrasound transducer assembly as claimed in claim 1, wherein theisolation layer comprises silicon oxide or a polymer material.
 7. Amethod for manufacturing an ultrasound transducer assembly forintravascular ultrasound systems, comprising the steps of: providing asilicon substrate having a substrate element portion including at leastone ultrasound transducer element for transmitting and receivingultrasound waves and including electrical connectors for electricallyconnecting the transducer element, forming a trench in the siliconsubstrate laterally separating the substrate element portion from anintermediate substrate portion surrounding the substrate elementportion, filling the trench with an isolating material different fromthe silicon substrate material to form a side layer connecting a toplayer and a bottom layer of the substrate element portion to each other,and removing the intermediate substrate portion surrounding the sidelayer to separate the substrate element portion from the siliconsubstrate so that the isolating material electrically isolates thesubstrate elements from each other.
 8. A method as claimed in claim 7,wherein a plurality of substrate element portions are laterallyseparated by a plurality of trenches and wherein the intermediatesubstrate portions between the substrate element portions are removed toseparate the substrate element portions from the silicon substrate.
 9. Amethod as claimed in claim 7, wherein the trenches are formed verticallyfrom a first side of the silicon substrate and the intermediatesubstrate portion is removed from a second side of the silicon substrateopposite to the first side of the silicon substrate.
 10. A method asclaimed in claim 7, wherein the intermediate substrate portion isremoved by an etching process and the side layer forms a lateral etchstop layer for the etching process.
 11. A method as claimed in claim 7,wherein a flexible layer is formed at a portion overlaying theintermediate substrate portion between the two substrate elementportions to form a flexible connection between the substrate elementportions.
 12. A method as claimed in claim 7, wherein the siliconsubstrate comprises a first and a second silicon layer disposed on topof each other separated by an intermediate layer, wherein the substrateelement portion is formed in the first silicon layer and theintermediate layer forms a vertical etch stop layer.
 13. A method asclaimed in claim 7, wherein the intermediate layer is patterned by anetch mask and an etch process in order to expose the intermediatesubstrate portion surrounding the side layers.
 14. A method as claimedin claim 7, wherein the trench is filled with an isolation material toform the side layer as an isolation layer.
 15. An ultrasound transducerfor vascular ultrasound systems, comprising an elongated probe includinga tip and an ultrasound transducer assembly as claimed in claim 1 foremitting and receiving ultrasound waves.
 16. The ultrasound transducerassembly as claimed in claim 2, wherein a central silicon substrateelement of the plurality of silicon substrate elements form at least aportion of a support element.
 17. The ultrasound transducer assembly asclaimed in claim 2, wherein an outer silicon substrate element of theplurality of silicone substrate elements is coupled to a lower supportelement, wherein an upper surface of the lower support element iscoupled to a lower surface of the outer silicon substrate element.